Why won't the compositor backdrop show the current composite result?
The issue stems from having rendered the animation as a series of .pngs, then loading the .pngs into the video sequence editor to test how the finished work looked.

If Blender has a video/image strip loaded into the VSE, it will not render an image to the Render Layers as it knows that the VSE strip will ultimately overwrite this information.

Instead, the Render Layers node in the compositor will appear empty or blank, as in the image at the bottom of my question.

Hiding the strip in the VSE will not solve this problem. Either
1) in the Render tab of the Properties Editor, open the Post Performance panel and untick 'sequencer' or
2) delete the image strip in the Video Sequence Editor.

Hitting render should now cause your Render Layer to show your expected image.

Annoying, and not especially evident to the user, but it makes sense. =)

I found my solution on this page: http://blenderartists.org/forum/showthread.php?274922-Camera-Tracking-render-layers-blank-source

Happy Blending!

--Rev